Slag is a by-product of metal production processes conducted in metallurgical furnaces. Although the composition and quantity of slag produced is highly dependent on the specific process, slag typically comprises a mixture of metal oxides with silicon dioxide, and is produced in amounts ranging from roughly 10 percent to several times the amount of metal produced by the process.
During metal production, slag is present in the metallurgical furnace in molten form. The temperature of the slag depends on the process, but can be on the order of about 1500-1600 degrees Celsius. Molten slag is periodically tapped from the furnace and is typically allowed to air cool and solidify, with the heat being lost to the environment.
Solidified slag can be processed into a number of commercial products, such as concrete blends or fibrous insulation material, but large quantities of slag continue to be discarded as waste. There has been recent interest in the use of granulated slag as a proppant in oil and gas production, or as roofing granules. However, the costs involved in transporting, grinding and/or re-melting solidified slag for the production of such materials can make these uses economically unattractive.
In addition, the processes used to convert slag to products such as proppants or roofing granules are not entirely satisfactory, and can result in materials which do not meet one or more product requirements. For example, proppants are required to have specific properties of strength, size, shape and composition. It is difficult to produce slag granules having all the required properties of proppants while preserving the economic benefit of using slag, rather than other starting materials.
There remains a need for simple, economically feasible processes for converting slag to commercially viable granular products, and for granular products such as proppants and roofing granules produced by such processes.